Rectifiers



tion. fairly satisfactoryperformance, but it is possible Patented Dec.18, 1945 UNITED j STATES PATENT oFFrcE RECTIFIERS David Rau, Kirkwood,and Carl E. Peters, Clayton, Mo., assignors to B. L. Electric Company,

St. Louis, Mo., a corporation of Missouri No Drawing. Application July29, 1942, Serial No. 452,804

9 Claims. (01. 175366) This invention relates to improvements inrectifierss More particularly the invention relates to improvements inmethodsof making rectifiers.

. current fiow in the other direction. These materials are said topossess current rectification prop- I I by use of the present inventionto materially increase the efliciency of selenium rectifiers, It is,

therefore, an object of the present invention to provide a moreeflicient selenium rectifier.

Th invention attains greater efliciency in selenium rectifiers byproviding a film or layer on the surface of the selenium. This layeris-then covered by an electrode, and thereby permanently sealed betweenthe selenium and the electrode. Y

This film or layer increases the resistance tov current flow in thereverse direction without materially decreasing the fiow of current inthe conducting direction, and thereby increases the efficiency of therectifier. It is, therefore, an object of the invention to provide-afilm or layer of material between the selenium and the electrode erties,and where an electrode is placed in contact rectification propertiesinherent in some semiconductors. V

In rectifiers. that have a semi-conductor-as an integral part thereof,the current rectification occurs at, or in the neighborhood of, one ofthe in contact therewith, that increases the resistance to the flow ofcurrent in the reverse direction without appreciably increasingtheresistance to the flow of current in the conducting direction.

Other objects and advantages of the invention will become apparent tothose skilled in the art from an examination of this description.

For the sake of brevity and clarity, this description is limited to anexplanation of how the invention may be used-in improving the currentrectification properties of selenium, but it is to be surfaces of thesemi-conductor. This surface is the surface in contact with theelectrode that is associated with the semi-conductor. The invenject ofthe present invention to improve the cur-' rent rectification propertiesof a semi-conductor by coating one surface thereof.

. One of the semi-conducting materials that is commercially acceptablefor use in rectifiers is a selenium, and this material inherentlypossesses fairly good rectification properties. Selenium will transmitcurrent relatively freely in the con-. ducting direction, but will offerconsiderable resistance to the flow of current in the reverse direc-Rectifiers made from selenium will give understood that the invention isnot limited to this particular method or ,to selenium rectifiers, andthe invention will be defined by the appended claims.

In improving the current rectification properties of selenium, theinvention forms a film or layer on the surface of the selenium after theselenium has been converted from the vitreous to the desired crystallineor metallic state. Selenium is usually supported on a base plate and isusually applied to the base plate with heat. As a result of thisheating, the vitreous selenium is attached to the plate. Before it canhave the desired current rectification properties, the selenium must beconverted to the crystalline ormetallic state by heat or by heat andpressure. After this has been done, the selenium may be immersed in asodium aluminate bath that will provide the desired coating.

The electrical characteristics of the selenium plate will depend uponthe length of time the selenium plate is immersed in the bath, upon thestrength and composition of the bath, and upon the temperature of thebath. The strength and composition of the bath and the length of timethe selenium plate must be immersed in the bath are interdependentfactors, and therefore a strong bath requires a lesser time'than a weakbath the precipitate.

would require. The temperature ofthe bath also is a factor indetermining the time of immersion.

Certain grades of commercial sodium aluminate solutions may be used butit is possible to make up sodium aluminate solutions that are better forthis work than the commercial solutions would be. A very practical andcommercial bath, but by no means the only bath that can be used, isobtained by dissolving 20 grams of .sodium hydroxide in 100 r'cc. ofwater, and then adding 20 grams of aluminum hydroxide. and grams ofRo'chelle salts. This bath is particularly desirable because it willordinarily remain in solution, but will readily form a precipitate onthe surface of a layer of selenium immersed in it. The Rochelle salts donot have any par-.

ticular utility other than to stabilize the solution to preventprecipitation of aluminum hydroxide. Sodium aluminate is a relativelyunstable compound that tends to form a precipitate. Such precipitationis desirable at the proper time because it gives the desired layer onthe surface of the selenium plate, but such precipitation in thecipitation in the absence of selenium would mean that the supply ofsodium aluminate would have to be renewed quite often. This would slowdown the manufacturing process involved and would also increase itscost. A making the bath strongly alkaline, we can eliminate most of theprecipitation that would occur in the absence of the selenium withoutretarding precipitation in the presence of the selenium. Furthermore, wehave found that the addition of Rochelle salts will materially assist inmaintaining the sodium aluminate in solution until the selenium isimmersed therein. This is very important because it adapts this methodto production line work and permits uniformity and standardization ofthe product.

When the selenium is immersed in the heated bath, a precipitate forms onits surface. This precipitate possibly contains sodium aluminate,probably the hydroxides of aluminum and some aluminum oxide. Thisprecipitation probably results from .a reduction of the alkalinity inthe neighborhood of the selenium due to the attacking of the selenium bythe alkaline components of the bath. We probably have a temporaryreduction of the alkalinity in the area immediately adjacent the surfaceof the selenium, and this reduction in alkalinity permits the formationof If the selenium were removed from the bath and washed, theprecipitate could be removed to a great extent, and it is, therefore,desirable to place the selenium in a hot rinsing bath of water. Thisrinsing bath sets the precipitate and also halts the chemical actionstarted in the original bath. After the selenium plate has been rinsedit is dried and is ready to receive the electrode that is to be placedin contact therewith.

The provision of this precipitate is quite useful as the following tableclearly shows. In making this test three untreated selenium plates werecompared with three treated selenium plates. The treated plates showed aslight increase in voltage in the forward direction of current butshowed a disproportionately greater increase in the inverse direction ofcurrent flow. This enabsence of the selenium is not desirable. Pre- Wehave'found that by minum hydroxide.

ables the selenium plate to operate much more efficiently as arectifier.

Voltage at Forward Inverted direction direction +.080 .008 amp. amp.

Untreated plates:

Whereas the description has described a preferred manifestation of theinvention, it is obvious to those skilled in the artlthat variouschanges may be made in the manifestation of the invention without.affecting the scope of the invention.

What we claim is:

l. The method of improving the current .rectif'lcation properties of theselenium in selenium rectifiers that comprises the forming of a layer ofsodium aluminate and sodium and aluminum hydroxides on one surface ofthe selenium.

2. The method of forming a layer on a selenium electrode that comprisesthe immersing of the selenium electrode in a heated bath of sodiumhydroxide, aluminum hydroxide and Rochelle salts.

3. The method of forming a layer on a selenium surface that comprisesthe immersing of the selenium in a bath of sodium hydroxide and alu- 4.An improved selenium rectifier comprising a selenium surface, anelectrode, and a layer of sodium aluminate disposed therebetween that isin direct contact with said selenium surface.

5. A method of making an improved selenium rectifier comprising theimmersing of the metallic selenium coated base plate in a solution ofsodium aluminate that is maintained at a temperature below or at theboiling point.

6. The method of producing an improved selenium rectifier comprising theimmersing of the selenium coated base plate in a solution of sodiumaluminate which is maintained at a temperature slightly below itsboiling point, rinsing the selenium in hot water and placing anelectrode in contact with the selenium surface.

'7. The method of forming a layer on a selenium electrode that comprisesthe immersing of the selenium electrode in a heated bath of sodiumaluminate and Rochelle salts.

8. The method of forming a layer on a selenium electrode that comprisesthe immersing of the selenium electrode in a bath of sodium aluminate.

9. The method of improving the current rectification properties of asemi-conductor, that comprises the immersing of the semi-conductor in abath of a soluble aluminate of predetermined strength for apredetermined length of time.

CARL E. PETERS. DAVID RAU.

